This tiny mouse thrives where humans can barely breathe

At the top out of a 6,739-metre Andean volcano, where the send is lean, temperatures ar almost ever below freezing and humans struggle to survive, a tiny leaf-eared mouse has made itself at home — dining on toxic plants.

An international team that included researchers at McMaster University in Hamilton, Ont., has uncovered some of the biological tricks behind that feat. The study, published Thursday in the journal Science, found that high-altitude Andean leaf-eared mice can generate heat more effectively when oxygen is scarce. Genetic evidence also suggests they have adapted to process harmful compounds in their food.

The species, Phyllotis vaccarum, has been found on the summit of Volcán Llullaillaco, on the Chile-Argentina border. At that elevation, each breath provides about 44 per cent as much oxygen as at sea level.

"The environment is so barren that you can't imagine anything would live there.… To actually know that these mice live up there routinely and survive, it was pretty mind-boggling," said Grant McClelland, a McMaster biology professor and study co-author.

The species has the broadest known elevation range of any mammal, living from sea level along Chile's northern coast to Andean summits more than 6,700 metres high.

Researchers collected mice across that range, compared their genomes and tested highland and lowland animals under the same laboratory conditions. They measured heat production at oxygen levels simulating sea level, 4,300 metres and 7,000 metres.

All the mice lost some heat-producing capacity as oxygen declined. But the highland mice lost less than lowland members of the same species and a related lowland species — a potentially lifesaving advantage in freezing conditions.

That advantage matters because producing body heat requires oxygen to fuel energy-hungry muscles, McClelland said.

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The highland mice appear to gain an advantage from how those muscles produce and use energy.

"Because it's so cold at these elevations, another really important thing they need to be able to do is shiver to keep their bodies warm," said Graham Scott, a McMaster biology professor and co-author.

"And so the way in which muscles support the metabolism for shivering is a lot like the way that a marathon runner would support movement."

In a hind-leg muscle used for shivering, the highland mice's mitochondria had a greater capacity to turn oxygen and nutrients into energy than those of lowland mice. Mitochondria are structures inside cells that combine oxygen and nutrients to produce usable energy.

"One of the big things is that there's just more of them.… They're just packed full of mitochondria," Scott said. The mitochondria also have "a really high capacity to use lipid fuels."

That matters because lipids, or fats, provide a long-lasting source of energy for shivering.

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But some of the study's most surprising findings involved food.

Very little grows on the highest slopes, leaving the mice little choice in what to eat. Genetic analysis revealed signs of selection in genes that help detoxify harmful plant compounds.

"At these elevations, they really have to eat whatever they get," Scott said. "These mice have had to adapt to cope with eating these plants with toxic compounds in them."

For a mammal small enough to fit in a hand, surviving near 7,000 metres is an enormous achievement — and a reminder that scientists may still underestimate where life can flourish.

"Evolution never ceases to surprise us as biologists," said McClelland, "because any environment you look at on Earth, including those that seem to be completely inhospitable to life itself, you're probably going to find something that's found a way to survive and thrive in that environment."

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